The harmful gas treatment technologies using low temperature plasma include:    U.S. Pat. No. 4,954,320, entitled “Reactive Bed Plasma Air Purification”, granted to Joseph G. Birmingham, et al. on Sep. 4, 1990,    U.S. Pat. No. 5,236,672, entitled “Corona Destruction of Volatile Organic Compounds and Toxics”, granted to Carlos M. Nunes, et al. on Aug. 17, 1993, and    U.S. Pat. No. 5,609,736, entitled “Methods and Apparatus for Controlling Toxic Compounds Using Catalysis-Assisted Non-Thermal Plasma”, granted to Toshiaki Yamamoto on Mar. 11, 1997.
In the above related-art technologies, pulse power is used to generate low-temperature plasma or catalytic or ferroelectric beads are filled in the reactor to increase the reaction efficiency and to reduce secondary contaminants in gases that may be generated after reactions.
These technologies have been very rarely commercialized and used since aerosol byproducts generated during their processes may be attached to pipes to clog them or to degrade the electric characteristics of the reactor, thereby impeding continuous operation of processes.
Most reactors used in low temperature plasma processes suggested until now have a structure using pairs of electrodes, one being in the form of a cylinder and the other being in the form of a thin wire or a small diameter tube.
Although these reactors are similar to conventional middle or large-size ozone generators available in the market and thus have advantages in that heat generated in the reactors is discharged to the outside to reduce their operating temperature, they have problems in that their size is large compared to the flow rate of gas.
In some small-size ozone generators using the plasma generation principle, the reactor includes multi-plate electrodes. The most remarkable characteristic of reactors with the multi-plate or multi-cell electrodes, which are widely known in the art, is an increase in their operating temperature since it is difficult to transfer heat generated in the reactor to the outside, compared to reactors with a cylindrical structure.
Due to this characteristic, reactors with the multi-plate or multi-cell electrodes may be advantageous in a harmful gas removal process in which a process of oxidation and removal of byproducts attached in the reactor may be promoted or the rate of removal of harmful gases may increase as the operating temperature increases.
However, in a harmful gas treatment process in which air and exhaust gases containing a large amount of moisture and particulate matter are treated, in contrast to processes of the ozone generator that generates ozone from relatively clean air or oxygen, arc discharges may occur to cause a serious damage to reactors with a cylindrical structure.
Thus, there is a need to provide a non-thermal plasma (NTP) reactor including multi-plate or multi-cell electrodes capable of generating stable low-temperature plasma even in gases containing moisture and particulate matter.
The structures of electrodes for generating plasma are classified into a rod (or wire) to rod (or wire) structure, a rod to plate structure, and a plate to plate structure. The types of dielectric barrier design for stable plasma discharges without causing arc discharges are classified into a Dielectric Barrier Discharge (DBD) type and a packed bed type.
If the power of the plasma generator exceeds a specific level, the harmful gas decomposition capability of the plasma generator reaches its limit so that the harmful gas decomposition capability is no longer increased even though more energy is supplied, thus resulting in high power consumption.
In addition, if the applied voltage is too high, electrons concentrate locally in the plasma generation region to increase the possibility of causing arc discharges, thereby further reducing the efficiency.
Thus, dielectric electrodes for generating plasma are stacked in multiple layers at regular intervals to increase the reaction region where gases can be exposed to plasma, thereby increasing the treatment efficiency of exhaust gases or gases for reaction.
Various technologies of the structure and production of plasma reactors using multilayer dielectric electrodes have been developed due to such a variety of advantages.
The most recently granted or published technologies of plasma reactors using multilayer dielectric electrodes include:    U.S. Pat. No. 6,979,892, entitled “Laminated Co-fired Sandwiched Element For Non-Thermal Plasma Reactor”, granted to Bob Xiaobin Li, et al. on Dec. 27, 2005, and    Japanese Patent Application Publication No. 2005-188424, entitled “Plasma Generation Electrodes and Plasma Reactor” and filed by KONDO ATSUO, et al. and then published on Jul. 14, 2005.
In the U.S. Pat. No. 6,979,892, dielectric electrodes are manufactured by co-firing ceramic dielectric bodies, each including an embedded electrode with electrical conductivity, and the manufactured dielectric electrodes are arranged so that surfaces of electrodes with opposite polarities oppose each other to define gas passages at regular intervals between the electrodes and the arranged electrodes are then stacked and bonded together with spacers.
Each of the spacers and the dielectric electrodes manufactured through co-firing internally has a vertical electrical via at the center of a portion where they are bonded together. This increases the reliability of insulation to allow stable plasma generation in various use environments of the reactor, particularly exhaust gas environments exposed to gases including a large amount of moisture and particulate matter.
While each ceramic dielectric body (or each dielectric electrode) including an embedded electrode internally has a well-insulated electrical via, the ceramic dielectric body also has a vertical electrical via for an opposing electrode and thus it is fixed to both vertical connection stacks of opposite polarities.
This limits individual thermal expansion and contraction of each of the dielectric electrodes stacked at regular intervals in environments with serious changes in the temperature of gases, for example exhaust gas environments of vehicles or exhaust gas environments of high-temperature incinerators. The limitation may cause a very high thermal stress locally to the dielectric electrodes to break them.
The method of manufacturing ceramic dielectric bodies, each including an embedded electrode, through co-firing is very effective in achieving insulation of the electrodes to which a high voltage is applied. However, electrical vias, which electrically connect the electrodes, must be made of very expensive platinum since both electrical connection and insulation cannot be achieved under conditions such as the co-firing temperature. In addition, the temperature and atmosphere where the electrodes are stacked at regular intervals are different from those where the dielectric electrodes are manufactured. This difference results in a thermal mismatch between the bonding layers to cause a local thermal distortion/stress which may separate the layers.
In this technology, the exterior of each electrode for generating plasma is made of dielectric material for stable control of the high voltage applied to the electrode and thus to generate stable plasma and expensive metal is used to achieve both electrical connection and insulation of the dielectric electrodes as described above. However, the technology failed to solve the fundamental problems such as dielectric cracking, parting line gaps, and split gaps which may occur in use environments of the reactor with serious thermal changes.
In the Japanese patent application publication 2005-188424, one fixing end portion is provided to fix each pair of opposing dielectric electrodes and a free end portion is provided at the opposite side to allow the dielectric electrodes to deform due to their thermal expansion and contraction in use environments of the reactor with serious temperature changes, particularly in environments for removing exhaust gases of vehicles and harmful matter of incinerators. Although this technology can ease the thermal stress that may occur locally, it has the following variety of fundamental problems.
To complete a plasma reactor according to this technology, a fixing end portion of each of multilayer plasma reaction electrodes is fixed to a stack including support members through a surface pressure applied to fix the structure to a case. This plasma reactor has parting line gaps and split gaps from the beginning.
Even when the plasma reactor is installed in the case with the optimal shapes or arrangements such as the optimal interval between gaps and the insulation distance, it is difficult to maintain the shapes uniform if both the plasma reactor and the case that applies the surface pressure to it repeat thermal expansion and contraction as the temperature changes and the exhaust gas pressure is constantly and repeatedly applied to the reactor.
In the structure where the fixing end portions are fixed only by the surface pressure, separation of layers will not only be constantly increased but moisture and particulate matter may also be constantly introduced into the parting line gaps and the split gaps in an environment exposed to gases including a large amount of moisture and particulate matter. Thus, this structure is very inappropriate taking into consideration the environment of using high voltages.
To compensate for these problems, it is necessary to provide a greater insulation distance for the limited dielectric electrode area. However, this may make it more difficult to secure the reliability of insulation while making the insulation members and the stacking method more complicated.
That is, terminals connected to an external power supply are easily exposed to moisture and particulate matter in the case, thereby easily causing arc or abnormal discharges. From the viewpoint of their structures, such reactors will have a great weight and will also require high manufacturing costs.
For these non-thermal plasma (NTP) reactors, it is very important to achieve insulation of connections between electrodes for stable control of high voltages due to the characteristics of the NTP reactors.
On the other hand, due to their characteristics, the components of plasma reactors using high voltages are mostly made of ceramic dielectric material. Especially, when the dielectric components are exposed to an environment with rapid temperature changes of exhaust gases as with exhaust gases of engines, the dielectric components are vulnerable to rapid temperature changes so that cracking, breakage, layer separation, and the like occur in reaction electrodes, electrode connectors, and the like, thereby failing to achieve the purpose of the dielectric material for proper control of high voltages.
It is essential for the plasma reactor not only to have insulation properties but also to have a resistance to thermal shocks that is applicable to the use environment of the plasma reactor.
From the viewpoint of material of each component of the plasma reactor, the resistance to thermal shocks can be increased by improving the thermal and mechanical properties of the material. However, selection of the material may be limited when the selection is made from materials with good electrical characteristics (especially, insulation performance and dielectric strength) that satisfy requirements of the durability, the plasma generation efficiency, and the like.